Some mice, if they get the chance, will drink alcohol more frequently than others, Polish scientists have demonstrated in an experiment. By studying mouse brains, the scientists hope to identify why some people are more prone to addiction than others.

What makes one individual more susceptible to addiction than someone else? Trying to solve this puzzle are scientists from the Nencki Institute of Experimental Biology of the Polish Academy of Sciences in Warsaw, led by Katarzyna Radwańska, Ph.D.

The scientists conducted a 95-day experiment during which they studied how keen mice were to take a sip of alcohol. The experiment showed that some mice were more eager than others. The tests may make it possible to better understand processes occurring in the human brain in terms of the degree of risk of developing alcohol addiction.

The scientists placed the mice in special cages where they administered drinking water and alcohol to them—in bottles placed in the corners of the cage. Each animal was marked. As a result, the scientists knew exactly which mouse was in a specific corner of the cage, and how much alcohol and how much water it drank. They could also activate a door that blocked off the bottle with the alcohol and check whether or not the mice were trying to gain access to it.

These animals were eager to drink alcohol, but it is difficult to say whether or not they developed an addiction, Radwańska says. In humans, this can be determined during an interview with a patient. If a psychiatrist wants to determine whether his or her patient is addicted to alcohol, they will ask specific questions: how long has the patient been drinking, does this affect their private and professional life, and whether they have health problems.

It turned out that a mouse desperate to drink alcohol could sit and keep knocking on the door with its nose many times over. And once it got a hold of the bottle, it drank for five seconds. A mouse that did not care much for alcohol would refrain from pushing on the door and go for water instead, Radwańska says. By depriving mice of access to alcohol, the scientists wanted to simulate alcohol withdrawal. After a short break, the mice were again allowed to access the alcohol, leading to a kind of alcoholic relapse. Just like alcoholics, some mice began to drink a lot, Radwańska says. One of the mice drank more than 12 grams of alcohol per kilogram of body weight in one day without any additional incentive. “That’s a lot because, after an injection of 4 grams per kilogram of body weight, a mouse can lose consciousness,” Radwańska says.

For each mouse, the scientists calculated an addiction index on the basis of which they classified the mice into two groups: those drinking a lot and those that did not care much for alcohol. According to Radwańska, this classification reflects the natural differences in a population. The mice were from the same cage and had the same access to alcohol, and yet they behaved completely differently. With people the situation is similar.

The brain of a mouse, like the human brain, comprises nerve cells called neurons. Each nerve cell has a lot of branches, and each of them has projections called dendritic spines. Thanks to these projections, nerve cells can communicate. The scientists investigated the differences between the structures of these spines in mice who like to drink and those who drink less. They analyzed the density and size of dendritic spines in the area of the brain responsible for memory formation.

It turned out that these parameters are smaller in those mice that are attracted to alcohol. “We believe that the properties of these spines and changes occurring in them due to different experiences determine the fact that some people learn more easily than others, and that some become addicted, while others do not,” Radwańska says.